What causes these black artifacts?
As Cegaton said,


  Z figthing... you have two or more overlapping faces 


This happens when two or more faces overlap. When trying to fix this go edit mode TAB then press z to go into wireframe mode. The darker sections darker than the normal greys are where faces overlap. Follow those to the vertices and move the vertice. (you may have to move than one) 

References 

Cause of black artifacts in render?